[FuzzMutate] Split out FuzzerCLI library that doesn't depend on IR.

All llvm-project fuzzers use this library to parse command-line arguments.
Many of them don't deal with LLVM IR or modules in any way. Bundling those
functions in one library forces build dependencies that don't need to be there.

Among other things, this means check-clang-pseudo no longer depends on most of
LLVM.

Differential Revision: https://reviews.llvm.org/D125081